In a typical computer system, devices request services from system software by generating interrupt requests, which are propagated to an interrupt controller via multiple interrupt request lines. Once the interrupt controller identifies an active interrupt request line, it sends an interrupt signal to the processor. In response, the interrupt controller interface logic on the processor determines whether the software is ready to receive the interrupt. If the software is not ready to receive the interrupt, the interrupt is held in a pending state until the software becomes ready. Once the software is determined to be ready, the interrupt controller interface logic requests the interrupt controller to report which of the pending interrupts is highest priority. The interrupt controller prioritizes among the various interrupt request lines and identifies the highest priority interrupt request to the processor which then transfers control flow to the code that handles that interrupt request.
In a conventional operating system (OS), all the interrupts are controlled by a single entity known as an OS kernel. In a virtual machine system, a virtual-machine monitor (VMM) should have ultimate control over various operations and events occurring in the system to provide proper operation of virtual machines and for protection from and between virtual machines. To achieve this, the VMM typically receives control when guest software accesses certain hardware resources or certain events occurs, such as an interrupt or an exception. In particular, when system devices generate interrupts, the VMM may intercede between the virtual machine and the interrupt controlling device. That is, when an interrupt signal is raised, the currently running virtual machine is interrupted and control of the processor is passed to the VMM. The VMM then receives the interrupt and handles the interrupt or delivers the interrupt to an appropriate virtual machine.